The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Starting with FIG. 1, vehicles traditionally include a power plant, such as an internal combustion engine 102, that generates drive torque. The drive torque is transferred through a powertrain and a driveline 104 to a driven wheel or wheels 106, which propel the vehicle along a surface. The powertrain 104 often includes an automatic transmission 108 that is coupled to the engine 102 by a torque converter 110, which is a type of fluid coupling, which allows the engine 102 to spin somewhat independently of the transmission 108.
Turning now to FIG. 2, a typical torque converter 110 is made up of a turbine 200, a pump 202, a stator 204, and transmission fluid. The housing 206 of the torque converter 110 is bolted to the flywheel 208 of the engine, and thus turns at the same speed as the engine. The fins that make up the pump 202 of the torque converter 110 are attached to the housing 206, so they also turn at the same speed as the engine.
The pump 202 inside a torque converter is a type of centrifugal pump. As it spins, fluid is flung to the outside. As fluid is flung to the outside, a vacuum is created that draws more fluid in at the center. The fluid then enters the blades of the turbine 200, which is connected to the transmission by turbine output shaft 210. The turbine 200 causes the transmission to spin, which moves the vehicle. Since the blades of the turbine 200 are curved, the fluid, which enters the turbine 200 from the outside, has to change direction before it exits the center of the turbine 200. This directional change causes the turbine 200 to spin.
The fluid exits the turbine 200 at the center, moving in a different direction than when it entered. The fluid exits the turbine 200 moving in an opposite direction than one in which the pump 202 (and engine) are turning. If the fluid were allowed to hit the pump 202, it would slow the engine down, wasting power. Therefore, a torque converter 110 has a stator 204 to prevent this waste of power.
The stator 204 resides in the very center of the torque converter 110. It is connected to a fixed shaft in the transmission by stator output shaft 212. The job of the stator 204 is to redirect the fluid returning from the turbine 200 before it hits the pump 202 again. This redirection dramatically increases the efficiency of the torque converter 110.
In some cases, there can be a lock-up clutch, which can create a firm connection between the pump 202 and turbine 200. The clutch is usually only engaged when a speed ratio of 1:1 has been achieved between turbine 200 and pump 202.
Turning now to FIG. 3, strategies for delivering power directly from the crankshaft into an automatic transmission have ranged from a purely mechanical connection via a high clutch drum and shaft transmitted through a damper plate assembly, to an actual clutch apply, all taking place inside the torque converter's fluid coupling. The converter clutch apply method has been the strategy of choice among vehicle manufacturers. This strategy has gone through several changes through the years. Some previous strategies have used a simple ON/OFF solenoid 300 in conjunction with an encapsulated check ball assembly 302 at the tip of the input shaft. The solenoid 300 turns the clutch on and off while the check ball assisted in a controlled apply of the clutch.
In more recent strategies, a pulse width modulated (PWM) torque converter clutch (TCC) solenoid 304 is added to the system in order to enhance converter clutch engagement for improved fuel economy. A powertrain control module (PCM) provides a duty cycle to this pulse width modulated (PWM) solenoid 304, which in turn regulates the pressure in the TCC hydraulic circuit, allowing the torque converter clutch to apply gradually. As apply pressure is increased, slip is also increased proportionally. Therefore, the amount of slip that occurs during the apply is proportional to the duty cycle.
The construction of the PWM solenoid 304 is such that when the solenoid 304 is completely turned off, feed pressure (AFL) to the solenoid 304 is blocked at the solenoid 304. When the solenoid 304 is duty-cycled, it opens to a circuit that allows pressure to act on the isolator valve 306. This increases the spring tension acting on the TCC regulator valve 308, which then increases regulated TCC apply pressure. As the duty cycle decreases, the regulated apply pressure decreases. As the duty cycle increases, so does the regulated apply pressure. As mentioned above, more pressure equals less slip and visa versa. The relationship between fluid apply pressure and input of the pressure control solenoid is essentially linear, and can be described by the following equation:y=mx+b; where y is fluid apply pressure, a is gain of the regulator valve, x is input of the pressure control solenoid, and b is offset of the solenoid spring.